What happens if I press the film rewind button early?

I don't know about this particular model, but in all my film cameras advancing the film (by pulling the film advance lever) resets the film release lock. Depress shutter with cap on, then advance again. You should see the winder turn, indicating the film is advancing. That should be enough to keep you shooting. (I'm assuming from your post, that you don't have a motorized drive). You won't lose the last frame provided you have not rewound the film at all.

EDIT: Corrected as per comment below (whuber)

It depends on how far it rewound and if it rewinds frame at a time or not. Film doesn't have frames, it's just one long continuous strip and each advance pushes it far enough ahead to avoid exposing over the last image. If the rewind has disengaged the film without rolling it back frame by frame, then you will have to advance a frame or two beyond where you were to ensure you are on unexposed film. If it supports going back exactly one frame, then you should be ok to simply take a photo with the lens cap on (black) and then advance as normal.